1. Field of the Invention
The present invention relates to a circuit for generating two-level differential PWM signals to be output to a class-D amplifier, and also relates to a method and apparatus for reducing pop noise.
2. Description of the Related Art
Nowadays there is a trend that a lot of audio amplifiers will have the form of class-D amplifiers. The reason is that they have a very high efficiency. This means that they don't consume so much power and they don't produce so much heat as conventional amplifiers like class-AB amplifiers for the same output power. Therefore, the class-D amplifiers can be kept relatively small.
A class-D amplifier uses pulse width modulation. The amplifier uses pulse width modulation (PWM) to convert an analog audio signal or a digital PCM signal to a pulse signal that is a function of the amplitude of the analog audio signal or the value of the digital PCM signal. In case of an analog system, the pulse width modulated signal is generated by modulating an analog input signal with a higher frequency carrier or a modulation signal. This can be, for example, a symmetric or saw-tooth triangular wave. The pulse signal can be easily amplified by using switches.
At the output of the class-D system, the high frequency digital pulse signal is filtered by a low-pass filter so as to filter out the high frequency switching noise, and the amplified input signal is retrieved.
FIG. 1 shows two-level PWM signals when the audio input is ‘0’. These signals are applied to the output stage of the Class-D amplifier.
FIGS. 2A to 2C give some examples of the class-D amplifier: FIG. 2A shows a circuit of a half-bridge class-D amplifier having one power source; FIG. 2B shows a circuit of a half-bridge class-D amplifier having two power sources; and FIG. 2C shows a circuit of a full-bridge class-D amplifier having one power source. Here, in FIG. 2C, PWMA′ is the inverted version of PWMA and PWMB′ is the inverted version of PWMB, and M1 and M3 are PMOS transistors and M2 and M4 are NMOS transistors. Here, inductors L and capacitors C except for capacitor C2 in FIG. 2A act as high frequency removal filters.
The class-D amplifiers as shown in FIGS. 2A to 2C, in comparison with other types of amplifiers, due to their specific driving mode, generate a so-called pop-noise at on/off of the power or at on/off of muting. Here, the pop-noise is generally used for a term meaning an unintended sound or a noise that can be heard as “a plosive sound” from a speaker. In this document, it is defined as a noise generated from a speaker or an electric noise occurring in a drive signal of the speaker, which occurs at on/off of the power of the amplifier or at on/off of muting and is not contained in a signal source.
FIGS. 3A and 3B show an example of a pop-noise in exemplary circuits of FIGS. 2A to 2C, and a result of a conventional countermeasure against the pop-noise. FIG. 3A shows a case in which the pop-noise is superposed on a differential input signal of the class-D amplifier at the start of driving the amplifier when no countermeasure is taken, the top diagram of FIG. 3A shows a differential input signal denoted by a thin line and an integrated input signal denoted by a thick line, the middle diagram shows a 2nd order integrated input signal denoted by a thick line, and the bottom diagram shows a filtered PWM signal denoted by a thin line (such as, a signal for driving a speaker itself) and an A-weight filtered signal of this speaker drive voltage denoted by a thick line.
FIG. 3B shows a case in which a pop-noise is superposed on a differential input signal of the class-D amplifier at start-up when one reduced-width pulse is incorporated as a pre-pulse, the top diagram shows an integrated input signal, the middle diagram shows a 2nd order integrated input signal, and the bottom diagram shows an A-weight filtered speaker drive voltage, which is denoted by a thin line.
FIG. 3A shows that when the signals PWMA and PWMB are applied on the output stage, an overshoot appears on the speaker voltage (Vsp). This results in a so-called pop-noise that can be heard by a listener through the speaker. Such a signal that is proportional to the audibility of human-beings can be made visible by filtering the speaker voltage with an A-weight filter. When the signals PWMA and PWMB are removed from the output stage a similar phenomenon will occur. In order to remove this overshoot, in case of a two-level PWM system, conventionally, the width of the first pulse (a pre-pulse) has been reduced to one-half of the width of the next pulse (duty cycle of 50%) (for example, refer to Publication of Japanese patent Laid-Open No. 2004-336765). FIG. 3B shows that applying this reduced-width pre-pulse can significantly reduce the overshoot.
This figure shows that, in comparison with the level of an A-weight filtered Vsp, when Vsp of no countermeasure has a peak-to-peak value of about 0.4 V, by adding the above-described reduced-width pre-pulses, the Vsp will have a peak-to-peak value of about 0.015 V (about 1/27).
In addition, in the above-described case, in a situation before the signals PWMA and PWMB are applied on the output stage, in other words, in a situation where the signals PWMA and PWMB are not applied on the output stage, transistors shown in FIGS. 2A to 2C are driven to be in an off situation.